Nitration 143useful for acid sensitive substrates.^117 Solutions of copper (II) nitrate and ferric (III) nitrate
in acetic anhydride have been used for the nitration of some reactive aromatic substrates.^118
A large variety of aromatics substrates have been nitrated with tetramethylammonium nitrate
and triflic anhydride in methylene chloride between− 78 ◦C and room temperature.^119
Metal nitrates supported on various acidic clays have been used as nitrating agents for some
reactive aromatic substrates in attempts to improve product isomer ratios.^120
4.3.4.8 Other nitrating agents
Alkyl nitrates in the presence of sulfuric acid^121 and Lewis acids,^7 ,^122 like SnCl 4 , AlCl 3 , and
BF 3 , have been used as nitrating agents. Nitrations in the presence of Nafion-H acidic resin
have also been reported.^123 Alkyl nitrates do not effect the nitration of aromatic substrates in
the absence of an acid catalyst.
Nitryl halides effect the nitration of aromatic substrates in the presence of Lewis acids,
although competing halogenation is a side-reaction in some cases.^124
Tetranitromethane in alkaline solution has been used for the nitration of some electron-rich
aromatic phenols and amines.^125
Olah^126 has conducted extensive studies into transfer nitration withN-nitro heterocycles.
Studies were conducted into the effect of the heterocycle and its counterion on reactivity
towards aromatic substrates.
4.3.5 Side-reactions and by-products from nitration
Spent acid disposal and the emission of nitrogen oxides are general consequences of indus-
trial nitrations with mixed acid. Side-reactions during nitration can also lead to undesirable
pollutants and by-products.^127 Substrates containing strong electron-donating groups like phe-
nols, phenol ethers and anilines, and some fused aromatics like naphthalene are all prone to
oxidation during nitration. Aromatic substrates containing alkyl substituents are particularly
susceptible to oxidation when nitrated. This is seen during the mixed acid nitration of toluene
to nitrotoluene where the crude product contains small amounts of phenolic by-products.^128
Phenolic by-products are often observed during the nitration of aromatic amines as a result of
diazotization with nitrogen oxides or nitrous acid present in the mixed acid.
Substituents like the nitro group protect against oxidation by withdrawing electron density
from the aromatic ring. However, even compounds with relatively electron deficient aromatic
rings will slowly oxidize on treatment with nitric acid or strong mixed acid at elevated temper-
atures. TNT and picric acid slowly form tetranitromethane on heating with concentrated nitric
acid via rupture of the aromatic ring. In fact, tetranitromethane is formed during the nitration
of dinitrotoluene to TNT and is present in the crude product. The harsh conditions needed for
the nitration of chlorobenzene to picryl chloride leads to the formation of some chloropicrin,
a highly toxic lachrymator.
1,3,5-Trinitrobenzene is present in crude TNT manufactured by mixed acid nitration and
results from methyl group oxidation followed by decarboxylation.^129 In fact, a convenient
method for the synthesis of 1,3,5-trinitrobenzene involves oxidation of 2,4,6-trinitrotoluene
with a solution of sodium dichromate in sulfuric acid, followed by decarboxylation of the
resulting 2,4,6-trinitrobenzoic acid in boiling water.^130 1,3,5-Trinitrobenzene is prepared from
2,4,6-trinitro-m-xylene by a similar route.^131 2,4,6-Trinitroanisole can be prepared from the